1. Field of the Invention
The present invention relates to integrated Voltage Controlled Oscillators (VCO) for RF applications. In particular, the invention relates to a transformer-based VCO for both phase noise and tuning range improvement in silicon bipolar technology.
The invention further relates to a Digital Video Broadcast-Satellite (DVB-S) receiver including a low noise block down-converter (LNB) for translating RF satellite signals picked up by a parabolic dish from a Ku-band to an intermediate frequency IF.
2. Description of the Related Art
In the last few years digital video broadcast services supported by geostationary satellites (DVB-S) have grown rapidly. In a DVB-S receiver the low noise block down-converter (LNB) translates the RF satellite signals picked up by a parabolic dish from the Ku-band (10.7–12.75 GHz) to an intermediate frequency IF ranging in the L-band (0.95–2.15 GHz).
The converted IF band is then sent to a decoder for tuning and digital demodulation.
LNB specifications are very challenging. See for instance the Technical Guide: EUTELSAT—“Information for Installers of HOT BIRD™ DVB-S Systems”—Vol. 1.
Indeed, an LO phase noise as low as −95 dBc/Hz at 100 kHz frequency offset from the carrier at 10 GHz has to be guaranteed.
In addition, an amplitude gain variation and an output P1 dB respectively within 8 dB and +5 dBm over the 2.05 GHz RF band must also be met.
Up to now, commercial LNBs have all been manufactured using discrete components. Usually, GaAs high electron mobility transistors (HEMT) and FET devices are used for the down-converter block, while dielectric resonator oscillators (DRO) are employed to generate the LO signal.
An integrated approach to LNB implementation would drastically reduce manufacturing costs. Moreover, the use of a PLL to generate the LO signal would eliminate the need for a testing phase to guarantee the accuracy of the oscillation frequency.
However, some technical problems are still to be solved to provide fully integrated solutions.
As is well know for those skilled in this specific technical art, voltage controlled oscillators (VCO) are circuit blocks of fundamental importance in radio-frequency integrated circuits.
The basic parameters for evaluating the performances of a VCO are: phase noise and tuning range.
The phase noise is an indicator of the spectrum purity of the generated oscillation; while the tuning range is the band in which it may be possible to control the oscillation frequency of the VCO.
In modern telecommunication systems the VCOs are requested to comply to more and more specific requirements in terms of phase noise and tuning range.
A conventional VCO structure realized by bipolar technology is shown in FIG. 1. The oscillation frequency of such a VCO is given by a resonator element LC wherein the capacitance C is the equivalent capacitance seen from the collector of each bipolar transistor Q1, Q2.
The capacitors CD and the resistors RB are used for providing a proper inverse biasing to the varactor diodes Cv.
For this biasing configuration, the parasitic capacitances CPk at the cathode terminals of the varactor diodes are connected to a virtual ground for the AC signals, so that they do not modify the oscillation frequency of the VCO structure.
While being advantageous under many points of view, this structure still suffers for some drawbacks.
Unfortunately, the capacitors CD, which are used to decouple the anode of the varactor diodes Cv, as well as the decoupling capacitors CDD, reduce the tuning range.
Moreover, the so-called thermal noise that is generated by the resistors RB increases the phase noise as well.
The biasing resistors RBB introduce a further contribution to the phase noise.
In this respect reference can be made to an article by: C. Samori, A. L. Laicata, A. Zanchi, S. Levantino, G. Cali, “Phase Noise Degradation At High Oscillation Amplitudes In LC-tuned VCOs,” IEEE Journal of S—S Circuits, vol. 35, no. 1, pp. 96–99, January 2000.
In this article it is disclosed the Single Sideband to Carrier Ratio (SSCR) dependence on the oscillation amplitude of a fully integrated LC-tuned voltage-controlled oscillator, manufactured in high-speed bipolar technology.
As the oscillation amplitude increases, the SSCR reaches a minimum and then steeply rises, setting a limit to the range where better performance can be traded against higher power dissipation.
This dependence is fully explained by taking into account that noise and disturbances modulate the phase delay due to the active elements.
Known prior art solutions still don't provide a fully integrated circuit